Variables in Vim Plugin

I write a vim plugin to learn some vimscript. I set the variables in the /autoload/plugin.vim like this:

let g:configpath = '~/home/username/.vim/bundle/testplugin/templates/'
let g:config = '/config.h'

Then I want to use it within a function:

function! testplugin#template(...)
    :e g:configpath  . a:1 . g:config   
endfunction

The goal is to open the file behind this path. a:1 is a user-given input because the subfolder is selectable by the user.

When I use echo g:configpath . a:1 . g:config the correct path will be print out to Vim but when I use :e to open the file Vim open a new file with the name:

"g:configpath . a:1 . g:config" [New]

How do I open the template file behind this path?

Solution

:help :echo and :help :edit_f handle arguments differently.

The former expects an expression, which is evaluated at runtime to produce the text that will be echoed:

:let $FOO = 'foo'
:let g:bar = 'bar'
:echo $FOO .. g:bar .. 'baz' .. eval('1+2')
foobarbaz3

The latter also does some evaluation but it is limited to expanding environment variables and the items documented under :help cmdline-special. Most notably it can't do concatenation:

:let $DIR = '/path/to/dir'
:edit $DIR/filename
:edit %<.stories.ts

If you want to build the filename programmatically, you have two options.

Use :help :execute:

:let filename = '/path/to/filename'
:execute 'edit ' .. filename
:execute 'edit ' .. <some expression that evaluates to a filename>

Use an environment variable:

:let $FILENAME = '/path/to/filename'
:edit $FILENAME

FWIW, autoloaded scripts don't seem to be the right place for defining global options. They are usually defined in plugin/foo.vim.